FIG. 1 shows a systematic block diagram of a power supply system for a flat panel display according to the prior art. As shown in FIG. 1, a power adapter 100 is configured to receive an input AC voltage Vin and convert the input AC voltage into an output DC voltage Vo. The output DC voltage Vo is provided to a DC-AC converter 101 (or inverter) and a DC-DC converter 102, respectively. The DC-DC converter 102 is configured to convert the output DC voltage Vo into a DC voltage with a lower voltage level for powering the control circuitry (not shown) located within the flat panel display. The DC-AC converter 101 is configured to convert the output DC voltage Vo into a high-frequency high-level AC voltage for illuminating the discharge lamps which functions as the backlight module for the flat panel display.
A typical power adapter for a flat panel display is configured to receive 110-V or 220-V input AC voltage and provide 12-V output DC voltage. Hence, the output DC voltage Vo provided by the power adapter 100 shown in FIG. 1 is generally a low-level DC voltage with the voltage level being 12V. The voltage level of the output DC voltage of the DC-DC converter 102 can be 1.5-V, 2.5-V, 3-V, 3.3-V, 4.5-V, 5-V, 6-V, 7.5-V, 9-V, or 12-V. However, the DC-AC converter 101 generally produces a high-frequency AC voltage as the desired power source for illuminating the discharge lamps located within the flat panel display. Therefore, the DC-AC converter 101 is responsible to convert the output DC voltage Vo having a voltage of 12V into a high-frequency AC voltage.
The DC-AC converter 101 is configured to convert a low-level DC voltage into a high-level DC voltage through the switching operations of the internal switches (not shown) and the voltage boosting operation of the internal high-voltage transformers (not shown). In this case, the voltage transformation ratio of the DC-AC converter 101 will become relatively large. Therefore, the DC-AC converter 101 would cause considerable power loss, which would deteriorate the conversion efficiency of the DC-AC converter 101. If the power adapter 100 can provide an output DC voltage with a higher voltage level for the DC-AC converter 101 to perform voltage transformation, the power loss caused by the DC-AC converter 101 can be reduced and the conversion efficiency of the DC-AC converter 101 can be enhanced.
FIG. 2 is a circuit diagram showing the power adapter 100 of FIG. 1. As shown in FIG. 2, the power adapter 100 includes a bridge rectifier 201 which is configured to rectify an input AC voltage into a full-wave rectified DC voltage and a transformer T21 having a primary winding Np21 and a secondary winding Ns21. The primary winding Np21 is configured to store energy from the input AC voltage Vin when a main switch S21 which is connected in series with the primary winding Np21 is ON and release the stored energy to the secondary winding Ns21 when the main switch S21 is OFF. The switching operations of the switch S21 are manipulated by a pulse-width modulator (PWM) 202. The power adapter 100 further includes a rectifier/filter circuit which is consisted of a rectifying diode D21 and a filtering capacitor C21 and connected to the secondary winding Ns21. The rectifier/filter circuit (D21, C21) is configured to perform rectification and filtration to the energy received by the secondary winding Ns21 so as to generate a desired output DC voltage Vo. The power adapter 100 further includes a feedback control circuit 203 which is configured to detect variations on the output DC voltage Vo and in response thereto issue a feedback signal to the pulse-width modulator 202 to enable the pulse-width modulator 202 to stabilize the output DC voltage Vo at a predetermined level.
As stated above, the output DC voltage Vo of the conventional power adapter 100 is a low-level DC voltage. If this low-level DC voltage Vo is provided to the DC-AC converter 101 shown in FIG. 1 for voltage transformation, the DC-AC converter 101 would cause a considerable power loss and deteriorate the conversion efficiency. Here, it would be an ideal solution to the above drawbacks if a power adapter capable of simultaneously providing a low-level output DC voltage and a high-level output DC voltage is devised. The present invention can satisfy these needs.